Embodiments of this invention relate generally to the field of solid oxide fuel cells and, more specifically, to anode structures for solid oxide fuel cells.
A solid oxide fuel cell (SOFC) electrochemically converts fuel into electricity. The solid oxide fuel cell has a solid oxide, or ceramic, electrolyte between a cathode and an anode. A conventional solid oxide fuel cell utilizes an yttria-stabilized zirconia (YSZ) electrolyte between the cathode and the anode. In general, the cathode reduces oxygen from the air into oxygen ions and passes the oxygen ions through the electrolyte to the anode. A conventional cathode material is lanthanum strontium manganite (LSM), or a similar material. The anode uses the oxygen ions to oxidize the fuel, which results in free electrons at the anode. The anode is typically a ceramic/metallic (cermet) material that includes YSZ as the ceramic and nickel (Ni) as the metal. By connecting an electrical load between the anode and the cathode (outside of the fuel cell), the electrons can return to the cathode, and the electrical generation cycle can repeat.
FIG. 1 depicts a schematic block diagram of a conventional SOFC system 10. The conventional SOFC system 10 includes a reformer 12, a sulfur trap 14, and a conventional solid oxide fuel cell 16. Sulfur can rapidly poison and deactivate the Ni—YSZ cermet anode of the solid oxide fuel cell 16. Since many fuels contain total sulfur levels that far exceed the levels that can damage the typical anode of the solid oxide fuel cell 16, the reformer 12 and the sulfur trap 14 are used to remove sulfur content from the fuel. Typical fuels which may be reformed by the reformer 12 include military fuels such as JP-8, JP-5, and NATO F-76. While military fuel sources are energy dense, these fuels are extremely complex in composition and contain a number of impurities and additives that present challenges for compact electrochemical power generation. JP fuels can contain as much as 3,000-4,000 ppm by weight sulfur, while Navy fuels (NATO F-76, etc.) can include as much as 10,000 ppm by weight.
The reformer 12 implements a reformation process to break down hydrocarbons (CxHy) from the fuel into reformate which includes synthesis gas (syngas) and hydrogen sulfide (H2S). The syngas includes hydrogen (H2) and carbon monoxide (CO), and also may include other components such as carbon dioxide (CO2) and steam (H2O). Although reformation by the reformer 12 reduces the sulfur content, typical sulfur levels for reformate from JP fuels is about 500-600 ppmv from an endothermic steam reformer and 300-400 ppmv from a partial oxidation (POx) reformer.
Hydrogen sulfide (H2S) content in the reformate of 2 ppmv (at 1000° C.) is known to poison the anode of a conventional solid oxide fuel cell 16. Additionally, sulfur poisoning increases the polarization resistance and over-voltage of the anode at as low as 0.5 ppmv (at 900° C.). This concentration of H2S is close to the equilibrium values measured at that temperature for the chemisorption of H2S to achieve full coverage on nickel steam-reforming catalysts. Because of this coverage, conventional Ni—YSZ cermet anodes are not sulfur tolerant.
Since the sulfur content of the reformate (i.e., syngas and sulfur) from the reformer 12 is significant enough to poison the anode, the reformate is directed through the sulfur trap 14 to remove the remaining sulfur content, leaving only the syngas. Many of the sulfur compounds present in these fuel streams are mildly reactive and, therefore, are relatively easy to remove. However, there are also considerable quantities of more complex sulfur compounds, including substituted thiophenes, which can be particularly difficult to remove via conventional liquid phase adsorption processes. In addition, even with the best possible liquid phase sulfur removal technology, the conventional solid oxide fuel cell is not capable of accommodating sulfur in the fuel stream due to intermittent malfunctions of the sulfur trap 14, or another sulfur removal system. In other words, occasional sulfur slip is anticipated. Additionally, the adsorption of liquid phase sulfur removal materials utilizes relatively large amounts of material, which increases the size and resources of the conventional SOFC system 10.
In contrast to the conventional Ni—YSZ cermet anodes, other conventional SOFC devices use a doped ceria (cerium oxide (CeO2)) anode or a copper-ceria (Cu—CeO2) anode. While ceria provides sulfur tolerance and some avoidance of coking if there is hydrocarbon slip, ceria is a mixed conductor in a fuel atmosphere and has low electronic conductivity. Thus, ceria alone does not provide a low polarization loss for a high performance anode. Infiltration of cerium nitrate (Ce(NO3)3) into a conventional Ni—YSZ cermet anode may show some level of tolerance to the presence of H2S.